The present invention relates generally to a method of fabricating automotive driveshafts and more specifically, to a method of attaching metal end-fittings such as an automotive U-joint yoke and splined tube shaft to tubular shafts which rotate at speeds and transmit torque and axial forces such as when in use as a vehicle driveshaft.
In general, a vehicular driveshaft transmits torque from a transmission to an axle to drive selected wheels of a vehicle. A driveshaft operates through changing relative angles between the transmission and the axle. Furthermore, a driveshaft expands and contracts in response to road conditions when the vehicle is operated. To accomplish these functions, driveshafts include well known universal joints and slip joints connected to driveshaft tubes.
A driveshaft tube includes a hollow cylindrical portion of a desired length, oftentimes terminating at one end in a tube yoke. The tube yoke includes a pair of opposed arms for receiving bearing cups mounted on trunnions of a cross. The tube yoke, cross and bearing can be combined with an end yoke to form a universal joint. The opposite end of the driveshaft tube can terminate in a splined tube shaft designed to receive an end yoke. The opposite end of the driveshaft tube can also terminate in a second tube yoke. Tube yokes and driveshaft tubes are conventionally formed from steel and are attached to the driveshaft tube by conventional welding processes.
In order to reduce vehicular weight, obtain smooth operation and improve fuel economy, driveshaft components have been formed from lighter materials such as aluminum. Pure aluminum does not make driveshaft components of acceptable strength, but alloys of aluminum have adequate strength. While aluminum alloys have been an acceptable material because of their strength and lighter weight, problems have been experienced using conventional welding techniques with such components. For example, aluminum components have been weakened by heat generated and transferred to them during conventional welding.
For the attachment of end-fittings to metal tubes, many other techniques are available with varying degree of success. Among these other methods are the use of pins, rivets, bolts, adhesives and such mechanical methods as splines, keyways, polygon matching shapes, shrink fits and press fits. However, these attachment methods are not as economical as desired, particularly when applied to driveshafts of vehicles.
In use today, with limited success, is one recent innovation known under the trademark MAGNAFORM. This technology employs a very high electromagnetic-induced force to swage aluminum tube onto a fitting, as is commonly used for non-driveshaft applications. Unfortunately, the results of such a method for attaching end fittings to driveshaft tubes have been less than satisfactory. Magnetic forming requires a non-circular, force-transmitting shape to transmit torque between two rotating parts. Aluminum, which is typically used in driveshafts, is a notch sensitive material, and is subject to cracking where it is stressed by being deformed into shapes having relatively large contours.
Also, when torque is applied to the driveshaft in the vehicle, there is a small amount of slippage between the yoke and the driveshaft tube which produces a loud and irritating sound. This has resulted in a large number of consumer complaints. Besides that, magnetic pulse forming gives good mechanical strength results only as long as the torque is not too high. But with a high level of torque, as measured with fatigue tests, the life of the driveshaft is reduced considerably.
A large number of revisions have been made in order to attempt to solve those problems. Unfortunately, all of these have been unsatisfactory. There is therefore a need to provide a solution which permits the advantageous use of magnetic pulse fields for swaging a tube and the advantageous use of the welding process for joining the end-fitting and the aluminum driveshaft tube.
A known prior art method of pressure welding is based on the use of interaction of magnetic fields, produced by an inductor through which an impulse of high intensity current is passed. The parts to be welded are positioned in spaced relation at an angle therebetween and the method can be used for obtaining overlapping welded joints of thin-walled parts having different thickness and made from different materials without melting. This is described in U.S. Pat. No. 3,520,049, to Lysenko et al. This method is referred to as Magnetic Pulse Welding (MPW) and has been used in particular to weld the end of nuclear fuel rods and has also found application in other contexts in which the diameters of the parts to be joined are small (about 25 mm) and tubes made from mechanical strength metal. Diameters of parts to be welded can be larger (about 60 mm) if tubes are made from technically pure aluminum and have a wall thickness of about 1.5 mm.
The apparatus for MPW as used today in manufacturing has the same basic design as the apparatus for magnetic pulse forming. The main parts of each apparatus are a capacitor bank, inductor and high current switching device. The technological capability of conventional MPW apparatus is much less than what is necessary for magnetic pulse welding of driveshafts having tube diameter within the range of about 75 to 180 mm and wall thickness of 2 to 3 mm. Further, conventional MPW apparatus is not capable of magnetic pulse welding of end fittings with driveshafts made from high-strength aluminum alloys like 6061T.
An improvement in welding tubular parts of large diameter using MPW is described by Yablochnikov in "Apparatus for MPW Large Diameter, Thin-Walled Pipes"; Avt. Svarka, 1983, No. 4 pp. 48-51, 58. That apparatus, named the Arc Magnetic Pulse Equipment (AMPE) has two main features: first, using a special type of inductor and, second, using a special vacuum switch which has closely-spaced ring-like electrodes that are positioned close to the inductor.
Between the electrodes there are insulators and a metallic housing. The contact surfaces of the insulators, the metallic housing and the electrodes are hermetically sealed to create a closed discharge chamber which is evacuated by a vacuum pump. Due to those features and extra-low inductance of the system connection bus bars, AMPE has minimal loss of energy in the process of discharge.
In principle, AMPE should permit tubes as large as a driveshaft to be welded using MPW, but there appear four problems which must be solved before this technology can become valuable from a manufacturing point of view. The first problem is the destruction and contamination of insulation elements of inductor by the powerful cumulative jet which flows axially along the welding surfaces (i.e., axially of the driveshaft tube) during the welding process. This cumulative jet is produced in the process of collision welding of metal when the impact velocity is high enough. The second problem is the low strength of the welding joint between high-strength aluminum alloy tubes and the end fitting if the latter is made from steel. The third problem is the possibility of premature breakdown of the vacuum switch. And the fourth problem is a long cycle time and resulting low productivity of AMPE. The last two problems are connected and contradictory to each other.
In the process of MPW welding, the surfaces of metal approach each other at an angle and collide with high relative velocity. The welding surfaces usually have oxide films and contaminants. To get a strong joint or weld, it is necessary to clean this contamination from the welding surfaces. In the process of MPW in the area where the surfaces collide with each other at high velocity, the cumulative jet includes material from the surface sheets and contaminants from the collision surfaces. This material carried with the cumulative jet acts to clean the welding surfaces.
The cumulative jet has supersonic velocity and creates a loud sound like thunder if allowed to escape to the atmosphere. If the cumulative jet is restrained, and reflected from obstacles such as the shoulder of the end fitting or the surfaces of tooling, and directed toward the insulation elements of inductor, then the cumulative jet can create problems. In such a case, the insulation elements can be contaminated and can be destroyed within a short number (perhaps less than 100) of welding cycles. Obviously this is unacceptable in a manufacturing process because breakdown of the inductor is possible.
As a result of the problems described above, welding using MPW has not yet been found to produce high quality welding joints between driveshaft tubes and end fittings if the driveshaft tubes are made from high-strength aluminum alloys like 6061 and any related temper, and the end fittings are made from middle carbonic steel like EMS-40. The physical reason for this is not known yet. But it is highly desirable in the manufacture of driveshafts to find a method to allow MPW of aluminum driveshaft tubes with split fitting because those fittings can only be made from steel.
The problem of eliminating the aforementioned self breakdown of the switch is a basic problem in the technique of high pulse current and strong magnetic fields. This problem becomes especially complicated if the amplitude of the current achieves a level of one mega-ampere or more, if the energy of the pulse is 40 kilojoules or more, if the charge transfer is 10 coulombs or more, and if the frequency of pulses more than one per minute.
Any high current switch must be able to withstand the working voltage of the capacitor bank without spontaneous breakdown. The switch should also have low inductance and inherent resistance. Further, the switch should have sufficient current throughput capacity, charge transfer and long service life. Depending on the actual conditions, to these main requirements are added others such as ease of linkage with the other components of the discharge circuit, quiet running, and a sufficiently narrow interval between discharge cycles. For magnetic pulse welding of a driveshaft, it is especially important to have such properties as a working switch with a narrow interval between discharge cycles and without spontaneous breakdown--contradictory requirements, especially for vacuum switches. The reason for the first of these properties (narrow interval) is the necessity to achieve highly productive output for driveshaft. The reason for the second (without spontaneous breakdown) is the impossibility of repairing the driveshaft in case of failure of the welding operation. This is a critical difference between the processes of magnetic pulse forming and magnetic pulse welding. The failure of the magnetic pulse forming operation can be corrected by using repetition of the discharge pulse. But the failure of the magnetic pulse welding operation cannot be corrected by using repetition of the discharge pulse because the first pulse changes or eliminates the gap between the welded surfaces, the value of which is very critical for the success of the MPW process. Failure of MPW results in an irrecoverably useless driveshaft tube. It is obvious that a long service life is also necessary for MPW driveshaft under manufacturing conditions.
For welding driveshafts using MPW only two types of inductor can be used. The first has a massive high-strength single-turn coil, the disadvantage of which is a gap between the leads, resulting in a nonuniform magnetic force field, and thereby providing a non-uniform weld. The other, preferred type of inductor has high strength coil comprised of a number of generally flat, closely packed but spaced-apart, nearly circular or annular electrical conductor strips, as disclosed in U.S. Pat. No. 4,129,846 to Yablochnikov. This type of coil provides a uniformly azimuthal distribution of the magnetic field and is used in the conventional AMPE process. To weld driveshafts using MPW, both types of inductors demand very high currents (1 to 2 mega amperes and higher) and a high energy of pulse (40 to 60 kilojoule and more).
The higher the amplitude of the current and the higher the energy of the pulse, the more complicated become the problems of switching that current's pulse. This problem becomes more and more complicated if the pulse current must be repeated with short intervals, as is necessary in an economical manufacturing process. The best results in switching the pulse current for a conventional AMPE process is a vacuum switch. It provides 2.0 to 2.5 current discharges in a minute, but this is not enough for economical manufacturing of driveshafts. The productivity must be at least 2 to 3 times higher.
The vacuum switch used in conventional AMPE has a gap between the electrodes of about 5 mm and is ready to switch if the residual pressure in the discharge chamber is lowered to about 10 to 20 Pascals. In this area of physical characteristics, the voltage of self breakdown of the switch increases inversely proportionally to the value of the residual pressure within the chamber as the pressure is being reduced. Unfortunately, this relation is true only if intervacuum surfaces of insulation elements are clean. But in the process of each switching step, the conditions on these surfaces are changing. High current discharge is accompanied by very intensive processes of electric erosion of electrodes and insulators. The products of erosion include vapors and small drops of metal from the electrodes. As a result of the deposition of these products of erosion on the elements of the vacuum switch, the switch is not capable of blocking the voltage developed across the capacitor bank if the charging starts too early.
The reason that it is difficult to maintain a switch at a high level of cleanliness to avoid premature discharge is as follows. After each discharge of the stored energy from the capacitor bank, the gaseous mixture from the vacuum chamber of the switch is evacuated by the vacuum pump. However, part of the metallic vapors and drops are deposited on surfaces of the insulators, and over time they form a coating on various insulating elements, and this consequently decreases the insulating properties. A complete understanding of the sophisticated physical processes inside the discharge chambers of the vacuum switches is not known, especially when the amplitude of the current reaches millions of amperes. But experiments found that a good vacuum in the discharge chamber is not sufficient by itself to prevent premature discharge.
During a welding cycle, the time required for recovery of the insulation properties of the intervacuum insulation and the time for charging the capacitor bank takes 80 to 90 percent of the entire working cycle of AMPE, which is typically 25 to 30 seconds. An additional disadvantage of the AMPE is that there is no guarantee that each cycle will work properly because a self breakdown is possible. A known solution to the problems of productivity and reliability of AMPE consists of separating the capacitor bank from the discharge circuit by means of special disconnectors after each switching during the time of pumping of the discharge chamber, and also measuring the breakdown voltage between electrodes. The processes of pumping the chamber and charging the capacitor bank can take place simultaneously. After achieving the breakdown and charging the voltages as necessary, the disconnectors are closed and switching can be done. The disadvantages of this solution are the sophisticated and large size required for the disconnectors. Also, a special hydraulic system controlled by the disconnectors is required if the design is based on a mechanical principle, and the use of mercury is required if the design is based on a liquid-metallic principle.
There is therefore a need to provide a solution which permits use of MPW for joining the various elements of driveshaft assemblies to each other, including attaching an aluminum driveshaft tube to an end fitting made of the same or different metals. This system should provide high productivity and reliability, and should avoid the complex design of the AMPE. Such a system should weld aluminum components of a vehicular driveshaft in such a manner so as not to damage the integrity or strength of the components or the final assembly.